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The caustic soda is dissolved in about half its weight f water,
and is added hot to the anthraquinone sulphoiiato <^>f~ sodium,
previously mixed into a paste with the potassivn"1"* _ chlorate
dissolved in about 50 c.c. of water. The niixture, wliioli forms
a stiff paste, is transferred at once to a small metal pressure tube
of steel or phosphor-bronze of the shape and dimensions shown

in Fig. 85-1 The mixture fills it
about two-tl^ircls full- ^ A sheet
of asbestos "cardboard iis inserted
between the body n.ri<:l the top
of the vessel, and the meUil lop
is then screwed firmly on. The
pressure tutoe is heat eel Tor three
hours in a paraffin- O1~ oil-bath,
so that the therm oineter in-
serted into tlie inner t vi lj>e, which
contains a little po.riiftln, regis-
ters 190200. The cliii-k violet
coloured mass, after cooling, is scraped out niicl di^'^^ted with
boiling water for an hour. Milk of lime is added until the
violet calcium alizarate is all precipitated. Tliis ca.n l^e ,'iscer-
tained in a small filtered sample by adding t little mi He of lime,
when no violet precipitate should be formed. The prcici pilule is
filtered at the pump and washed with boiling- water until the
filtrate is no longer red. The red filtrate contains a. little: mono-
hydroxyanthraquinone, which may be precipitated. !>>r hydro-
chloric acid. The calcium alizarate . 011 tlie filtor is sus-
pended in a large quantity of hot water, and decomposed hy
adding hydrochloric acid. The alizarin, which. sepn.ri.Ltos as an
orange, flocculent precipitate, is filtered cold, \vashed o.l^out eight
times with cold water, and finally dried and cryst^Llliisecl from
alcohol or preferably cumene. Yield, 10  i 5 grams.

The thickness of the metal is i cm.
FIG.  85.

Properties. Orange needles ; m. p. 28 c> - 290 ; sublimes
completely at 140 without decomposition ; soluble In alkalis
with a deep purple colour (sodium alizarate). . It is reduced to
anthracene on heating with dry zinc dust.
1 The apparatus was made for xu by West's Gas Improvement Co. , 3VZ ilos Platting,